Liquid crystals, i.e., materials with liquid crystalline properties, are "highly anisotropic fluids that exist between the boundaries of the solid and conventional, isotropic, liquid phase." D. B. DuPre, Kirk-Othmer Concise Encyclopedia of Chemical Technology, 1985, John Wiley & Sons, pp. 703-705. That is, materials that are in a liquid crystalline state exhibit anisotropic properties, i.e., properties that differ in different directions, such as thermal/electrical conductivity and light transmission. This liquid crystalline state, i.e., anisotropic state, exists between the boundaries of a solid crystalline phase and an isotropic liquid phase for a given material.
Materials that exhibit liquid crystalline properties typically have an elongated, narrow molecular framework. Examples of organic compounds that exhibit such properties are para-n-octyloxybenzoic acid, para-azoxyanisole, 4-(4'-ethoxybenzylideneamino)cinnamate, bis-(4'-n-heptyloxybenzylidene)-1,4-phenylenediamine, and the like. In materials such as these, the molecules exhibit three dimensional, long-range order, with respect to their centers of gravity, while in a solid crystalline phase. While in a liquid crystalline state, however, the molecules in these materials lack order in at least one dimension, while still maintaining significant long-range orientational order. This long-range orientational order is sufficient to impart solid-like properties to the fluid state. That is, the extent of molecular ordering and intermolecular forces in the liquid crystalline state of a liquid crystal is sufficient to impart properties of both a solid and a liquid.
Simply because a molecule is long and narrow, however, does not indicate that the material will possess liquid crystalline characteristics. There must be sufficiently strong forces between the molecules to retain an ordered molecular arrangement upon melting, for example. That is, the intermolecular forces must be sufficient to form regular solids, while at the same time the molecules must be free to move as they do in liquids.
Because the intermolecular forces impart both solid and liquid properties, liquid crystals are sensitive to external changes in temperature, pressure, electric fields, magnetic fields, and the like. Thus, they are extremely useful in devices that monitor such changes, and/or cause an event to occur upon such a change. Liquid crystals are therefore useful in sensors, switches, and shutters, for example.
Liquid crystals are characterized as either thermotropic or lyotropic. That is, liquid crystals are characterized by how the material forms a liquid crystalline state. The liquid crystalline state of thermotropic liquid crystals results from a phase transition from a solid to a liquid crystalline state upon heating. This occurs at the melting point of the material. The liquid crystalline state of lyotropic liquid crystals results from the action of a solvent on the solid. That is, the anisotropic state of a lyotropic liquid crystal exists in solution, whereas the anisotropic state of a thermotropic liquid crystal exists in a melt. The anisotropic state, i.e., the liquid crystalline state, of both a lyotropic liquid crystal and a thermotropic liquid crystal can be converted into an isotropic fluid at sufficiently high temperatures, and for a lyotropic liquid crystal upon dilution. For a thermotropic liquid crystal, this transition occurs at the "clearing" temperature.
Lyotropic liquid crystals can be used to prepare fibers by pulling the fibers out of the solvent. They can also be readily used in the preparation of composite materials. Thermotropic liquid crystals are advantageous and preferred to lyotropic liquid crystals because they do not require the use of an organic solvent and are generally easier to process. Thus, they can be used for a wider variety of products, particularly films and fibers.
Whether thermotropic or lyotropic, there are three distinctive structures that result from the dimensional and packing characteristics of the molecules. These structures are known as the smectic, nematic, and cholesteric structures, and often exhibit further subclasses of structures. A liquid crystalline material may exhibit one or more structures and/or structural subclasses. In thermotropic liquid crystals, for example, transitions between structural subclasses occur as the temperature is varied. These transitions are usually consistent with the gradual breakdown upon heating of long-range molecular order. Frequently, such transitions are reversible upon heating and cooling. Electric and magnetic fields can also induce such transitions.
Because polymeric materials, such as linear polymers, often adopt an elongated configuration, it was generally believed that they could also exhibit liquid crystalline properties. Many polymers, however, are thermally stable, insoluble, or decompose prior to forming a fluid. Thus, it has not generally been possible to demonstrate that polymers can exhibit liquid crystalline characteristics.
Polymers that do exhibit liquid crystalline characteristics, however, can exhibit either lyotropic, thermotropic, or both types of liquid crystalline characteristics, as can molecular liquid crystals. Polymers that exhibit thermotropic liquid crystalline characteristics do so at temperatures in a range between the melting point of the solid and the lower of either the clearing temperature or the decomposition temperature of the material. Such polymers are useful in the production of lightweight, ultrahigh strength, and temperature-resistant fibers. Thus, polymers are needed that exhibit liquid crystalline characteristics, particularly thermotropic liquid crystalline properties.
Polymeric materials, such as certain polyesters, polyethers, polyamides, polyisocyanates, polyphosphazines, and polysiloxanes, exhibit liquid crystalline properties, particularly lyotropic liquid crystalline properties. Polymeric poly(yne) materials containing certain transition metals in the backbone are also known to exhibit lyotropic liquid crystalline properties. For example, --[--Pt(PBu.sub.3).sub.2 --C.ident.C--C.ident.C----].sub.n --, --[--Pt(PBu.sub.3).sub.2 --C.ident.C--C.sub.6 H.sub.4 --C.ident.C--].sub.n --, and --[--Pt(PBu.sub.3).sub.2 --C.ident.C--C.sub.6 H.sub.4 --C.ident.C--.ident.C--C.sub.6 H.sub.4 --C.ident.C--].sub.n -- have been shown to exhibit lyotropic liquid crystalline properties. See S. Takahashi et al., Macromolecules, 12, 1016 (1979). Although these polymers are soluble and exhibit lyotropic liquid crystalline properties, they decompose before melting and thus do not exhibit thermotropic liquid crystalline properties.
Numerous organosilicon poly(yne) polymers are known. However, none have been reported in the literature to exhibit liquid crystalline properties. For example, silylenediethynylbenzene polymers of the general formula --[--(SiR.sub.2).sub.m --C.ident.C--C.sub.6 H.sub.4 --C.ident.C--].sub.n --, wherein m=1-2 and R=H, methyl (Me), and phenyl (Ph), are known. See, for example, Y. Ding and T. J. Barton et al., XXIII Organosilicon Symposium, Apr. 20-21, 1990, Midland, Mich.; R. J. P. Corriu, J. Polymer Science: Part C: Polymer Letters, 28, 431 (1990); H. Q. Lin et al., Can. J. Chem., 68, 1100 (1990); I. W. Shim et al., J. Organomet. Chem., 260, 171 (1984); and L. K. Luneva et al., Vysokomol. Soyed., A9, 910 (1967). These polymers contain the 1,4-diethynylbenzene unit in the chain, which can potentially impart liquid crystalline properties to the polymers. However, these polymers have not been reported in the literature to display either type of liquid crystalline property. Therefore, a continuing need exists for organosilicon polymers that can exhibit liquid crystalline properties.